How to deal with a jealous friend?

A friend introduced me to her best friend and he seems to be closer with me. We hang out a lot and now she's often rude to me. She’s always busy, so him and I have been hanging out while she’s away. When she sees us she’s often cold to me or won’t speak to me unless I speak to her. She treats him the same though and tries to isolate me whenever she gets a chance to. I’m not really hurt, it’s just awkward lol but it needs to stop. This is immature behavior and we’re all in our early 20s. I haven’t known either of them for very long. But they’ve known each other for years. Him and I have a playful "fuck you" type of banter, so when she's rude to me he playfully joins in without realizing she's serious. What do I do?

She is probably jealous, and doesn't know how to handle the new dynamic with you being the new person Not sure what to do here, it may dissipate if you add more people to the group - then she may see that you are not trying to make her jealous at all!